1. Field of the Invention
The present invention relates to a process for the production of a glass preform for an optical fiber and an apparatus for carrying out such a process. Particularly, the present invention relates to a process for the production of a porous glass preform in which non-deposited fine glass particles are prevented from depositing in to the inside of an exhaust duct and/or residing within the exhaust duct when the glass preform is produced by deposition of fine glass particles: The present invention also relates to an apparatus for such a process.
2. Description of the Related Art
Processes using vapor phase reaction for producing a glass preform for an optical fiber have been known. As examples of such a process, the inside chemical vapor deposition (CVD) process, the outside chemical vapor deposition process and the vapor phase axial deposition process are known. In these processes, a vaporous starting material for glass flows out through an oxyhydrogen burner nozzle and a flame hydrolysis reaction occurs there, whereby fine glass particles produced from the reaction are deposited in the form of a rod to obtain the porous glass preform for the optical fiber in a reaction vessel.
FIG. 2 schematically shows a prior art system for the production of the glass preform for the optical fiber. In such a system, the fine glass particles are produced from combustion of a starting material gas of glass such as SiCL.sub.4 gas, hydrogen gas and oxygen gas. The starting material gas flows out of the burner 2 in the reaction vessel 1, and the particles are deposited on the seed rod 4 as the porous glass preform 3. The seed rod 4 is grasped and rotated and also lifted by the chuck means 5 as the fine glass particles are deposited thereon.
In the above-described process, reactions occur according to the following reaction formulas in the reaction vessel in the case in which SiCl.sub.4 is used as the starting gas material of glass: EQU H.sub.2 +1/2O.sub.2 .fwdarw.H.sub.2 O+5.78 Kcal/mol EQU SiCl.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4HCl
As seen from the above formulas, heat of formation is generated due to the reaction of hydrogen and oxygen together with formation of SiO.sub.2 fine particles.
Usually, about 50 to 90% by weight of the fine glass particles produced according to the above reactions (SiO.sub.2 fine particles in this case) is deposited in the form of a column to produce the porous glass preform the balance, which is not deposited, is exhausted through an exhaust port 6 to the outside of the reaction vessel 1 by being entrained with an exhaust gas. Such fine glass particles which have not been deposited to form the preform in the reaction vessel will be, hereinafter, referred to as "non-deposited (fine) glass particles". The exhaust gas comprises, in addition to the non-deposited fine glass particles, the product gas materials (such as H.sub.2 O, SiO.sub.2 and HCl), and also comprises ambient air around the reaction vessel which comes from the suction port of the reaction vessel. The exhaust gas containing the non-deposited glass particles reaches the fine glass particle scrubber 9 through a connection pipe 7 and an exhaust duct 8, and the gas is separated from the fine glass particles in the scrubber 9. Then, the exhaust gas, which is free from the glass particles, is withdrawn from the system through the acid gas treatment facility 10 to the ambient atmosphere (usually to the outside of a clean room in which the reaction vessel 1 is placed). It should be noted that some fine glass particles remain in the reaction vessel 1 and attach to the inside surface of the reaction vessel; however, a ratio of such particles to the formed particles is very small.
As to the heat of formation due to the reaction of oxygen and hydrogen, usually about 60 to 90% thereof is exhausted with the exhausted gas so that the exhaust gas is heated to an elevated temperature.
It is well-known that the apparatus for the production of the glass preform for the optical fiber is placed in a clean room which is pressurized positively so as to maintain the cleanness in the room. In order to also maintain the positive pressure in the clean room, an amount of air exhausted from the clean room should be limited. Thus, generally THE apparatus for producing the glass preform for the optical fiber is arranged to minimize the amount of the exhaust gas which is withdrawn from the reaction vessel because the exhaust gas comprises the ambient air of the reaction vessel and is drawn out to the outside of the clean room. Since, for such a purpose of minimizing the amount of the exhaust gas, an exhaust gas speed through the exhaust duct is as low as about 7.+-.3 m/sec., fine glass particles which have not been deposited on the rod (i.e. the non-deposited fine glass particles) are very likely to deposit on the inside surface of the duct, thereby partially or completely blocking the duct. Due to the possibility of duct blockage, operation of the apparatus for the production of the glass preform for the optical fiber, hitherto, has to be stopped once per every two or three months so that the inside of the duct can be cleaned. Therefore, the productivity of the glass preform is reduced. Also, the blockage tendency of the duct makes an operation pressure in the reaction vessel unstable so that stable production becomes impossible. In addition, materials of which duct is formed are heated to an elevated temperature due to the heat of formation of the reaction between hydrogen and oxygen, and normal materials for the duct cannot withstand such an elevated temperature. Therefore, the duct should be made of special materials which are generally expensive, such as a heat resisting glass, Teflon and a high grade metal (for example, pure nickel, a nickel based alloy and so on).